1. Technical Field
The present invention relates to a nacelle and wing assembly for an aircraft, and more particularly to such an assembly with means to improve aerodynamic performance when operating at high angles of attack with the wing in a high lift configuration.
2. Background Art
It is quite common in present day aircraft designs to place the engine nacelle below the wing with the inlet of the nacelle being positioned forwardly of the leading edge of the wing. The design of modern, efficient transport aircraft has increasingly led to the use of high-bypass ratio engines which are considerably larger in diameter than equivalent low-bypass engines, but are highly desirable because of their significantly lower fuel consumption and reduced noise level. Forced by practical limits on landing gear, length and runway clearance, designers of new jet transport designs that employ such wing mounted, high-bypass ratio engines increasingly require much closer coupling of these larger engines to the wing than was the practice in the previous generation of transport designs.
Even with this close coupling of the nacelle, the nacelle/wing assembly is normally designed so that in cruise configuration the airflow around the nacelle causes relatively little disturbance to the airflow adjacent the wing. However, for the landing and/or takeoff mode, the wing generally has leading edge and trailing edge devices which are deployed to a high-lift position, and the aircraft is generally operating at a relatively high angle of attack. This results in increased interaction of the engine nacelle and wing flow fields, and particularly with closely coupled nacelles, this can degrade airplane performance. One of the most persistent and limiting problems relating to engine-wing interference is the reduction of maximum lift capability in the landing and take-off configurations. In the case of swept wing jet transports, this interference typically will result in the stall of the portion of the wing inboard of the nacelle at a much lower lift than would be the case with a smaller, less close-coupled engine. Occasionally, the interference will be severe enough that early stall will also be evident on the wing outboard of the nacelle; however, on swept wings typical of modern transport the inboard interference is generally much more severe.
A particularly damaging aspect of this problem is that the interference of this type is not affected by scale, so that while the lift producing capability of the wing alone typically increases with size (Reynold's number), the lift capability of the wing and nacelle remain constant (or is reduced) once interference related stall occurs. This makes small scale wind tunnel testing of such configurations most risky, significantly reducing the usefulness of one of the aerodynamicist's most powerful tools.
One prior art approach to identify the nature of the general problem of nacelle/wing flow interaction and provide an appropriate solution is disclosed in U.S. Pat. No. 3,744,745, Kerker et al, where there is shown a nacelle/wing assembly with a swept back wing and an engine nacelle mounted downwardly and forwardly of the wing leading edge. Mounted on the forward upper surface portion of the engine nacelle just behind the nacelle inlet are a pair of lift vanes. The text of the patent indicates that these vanes produce a downwash field between trailing vortices in the vicinity of the wing leading edge. It is further alleged that this field in turn reduces the high suction pressure peaks on the wing leading edge aft of the engine nacelle, tending, in turn, to delay the onset of airflow separation at the wing trailing edge forward of the flap. It is further stated that this delay in airflow separation permits the wing to generate more lift.
While it is generally known that vanes can be used to improve performance of the nacelle/wing assemblies in a high-lift mode of operation, these same vanes have an undesirable aspect in that for cruise mode of operation, they will normally tend to increase drag. Therefore, in general, it is desirable to size these vanes to alleviate the drag problem for cruise configuration, and yet to arrange the configuration and location of such devices to optimize their desirable effects in improving peak performance during the high lift/high angle of attack mode of operation.
However, the task of designing devices to modify the airflow about a nacelle/wing assembly is enormously complex. In the design of an aircraft, generally the overall configuration of the wing and nacelle are at a certain stage of the design fixed within certain limits. If an aerodynamicist had the luxury of unlimited time to test a particular nacelle/wing assembly in the wind tunnel, with unlimited budget, then he could embark on an extended experimental program of placing flow modifying devices of various configurations at various locations and eventually, by a lengthy process of trial and error, the overall design could be optimized. However, as a practical matter, where the entire airplane must be designed within certain budget limitations, such endless experimentation is not possible. Therefore, it becomes incumbent upon the aerodynamicist to analyze the performance and flow patterns of a given nacelle/wing assembly to determine which experimental avenues might be examined more advantageously to optimize performance. To conduct this analysis successfully with reasonable consistency requires not only an understanding of the aerodynamic phenomena which exist for a particular nacelle/wing assembly, but also what sort of flow modifying means might be employed successfully to be effective in resolving the problems associated with that particular phenomena.
In view of the foregoing, it is an object of the present invention to provide a nacelle/wing assembly with an appropriate flow control means, a method of making such an assembly, and a method of utilizing such an assembly, where, with the nacelle/wing assembly in a high lift, high angle of attack mode of operation, certain aerodynamic phenomena exists which are alleviated by effective use of certain flow modifying means.